(Reuters) - For the nearly 5 million people who live along the U.S. coasts from Maine to the Gulf of Mexico and the West Coast, rising seas fueled by global warming have doubled the risk of so-called once-a-century floods, according to a trio of environmental reports released on Wednesday.

These new reports - one from the non-profit group Climate Central and two others published in the peer-reviewed journal Environmental Research Letters - offer a detailed picture of where the most severe risks are along coastlines of the contiguous 48 states.

Based on 2010 U.S. Census population data and a fresh analysis of high tide lines by the National Oceanic and Atmospheric Administration, the Climate Central report's findings can be seen online at surgingseas.org

South Florida may be "indefensible" against floods caused by higher seas and the bigger storm surges that are expected to result, said Ben Strauss, an expert on ecology and evolutionary biology who is chief operating officer of Climate Central. He co-authored the two journal reports and the online report.

An estimated $30 billion in taxable property is vulnerable in southeast Florida alone, according to a preliminary independent analysis cited in the report.

In California, some places that have never seen severe floods could be vulnerable to them in the next decade or two, Strauss said.

Climate scientists maintain that people, businesses and infrastructure in low-lying coastal areas of the contiguous 48 U.S. states are vulnerable to sea level rise, and world sea levels have risen by 8 inches since 1880.

This rise in the world's seas is caused by the expansion of ocean waters as they warm and by the melting of glaciers and ice sheets, Strauss said. This is due to global warming fueled by the emission of heat-trapping greenhouse gases, he and other climate scientists have said.

Forecasts for sea level rise this century range from 2 to 7 feet, with most estimates centering around 3 to 4 feet, given a projected rise in global temperature of at least 2 to 3 degrees F (1.1 to 1.6 degrees C), Strauss said.

VULNERABLE ZONES

As seas rise, they amplify storm surges, the reports said.

Global warming will more than double the odds of once-a-century floods by 2030 for more than two-thirds of the 55 coastal locations considered in the analysis, the Climate Central report said. For a majority of the locations, warming triples the odds of century floods.

By 2030, storm surges combined with rising seas could raise waters to 4 feet or more above high tide lines at many locations, the reports said, noting that 4.9 million people live in 2.6 million homes in this vulnerable zone between the observed high tide and the top of expected flood waters.

Cities are likely to be hit hardest, Strauss said, with 90 percent of the impact projected to come in areas with extremely dense population.

In 285 coastal cities and towns, more than half the population lives below the 4-foot mark, the Climate Central report found. Florida has 106 of these at-risk municipalities; Louisiana has 65, New Jersey and North Carolina have 22 each, Maryland has 14, New York has 13 and Virginia has 10.

Florida is a special case because in addition to rising seas and storm surges, its geology and system of drainage canals pose complex problems. "Basically, south Florida in the long term is indefensible," Strauss said.

"A lot of the state is built on porous bedrock, bedrock that's like Swiss cheese," he said. "You can't practically build a wall to keep the sea out. The water will come up through the ground."

To prepare for the future, he said, researchers looked to the past, and compared current flooding patterns to what occurred before sea levels rose 8 inches, starting in 1880.

"Today's century storm would be a once-in-200- or 300-year event, or rarer if there had been no sea level rise from global warming over the last century," Strauss said. "The point is, we're already in the teeth of this."